1. Field of the Invention
The present invention relates to an apparatus and method for designing a sheet metal part using a computer aided design (CAD) system. More particularly, the present invention relates to an apparatus and method for designing a flat sheet metal part to be folded or bent into a user specified part (three dimensional part) having smooth, finished corners with no overlapping of faces or spaces between adjacent faces after bending.
2. Background Information
Typically, in order to create a finished folded or bent sheet metal part from a flat sheet metal part (flat), a user develops a three-dimensional (3-D) model of the desired finished sheet metal part on a computer system. From the three-dimensional model, a two-dimensional (2-D) model (flat sheet metal part) can be determined. The 2-D model shows bend lines necessary for folding the flat sheet metal part into the desired 3-D part using e.g., a die and punch, as well as the shape into which the flat should be cut prior to being bent. Then, using the 2-D model as a blueprint, the flat sheet metal part (flat) is cut to the specified shape. Subsequently, the flat is bent along the specified bend lines to create the desired finished part.
However, it is very difficult to design the flat such that smooth, finished corners result after bending because typically the part will have interference or collisions between faces which become adjacent after bending. The difficulty arises because the geometry modeling and the computation of the flat from the user specified 3-D part are complex. One reason for the complexity is that expansion or shrinking of the sheet metal occurs during bending which causes collision/interference to occur between adjacent bent surfaces. Adding to the complexity is the fact that different materials exhibit different expansion/shrinking properties. Consequently, as a result of the collisions the part will not bend properly or, if the sheet metal is thin, it will bend but warping will occur near the corners. Another problem is that surfaces (i.e., flanges) of the parts may overlap during bending and result in a rough transition between surfaces.
To solve these problems, traditional sheet metal part designers would design a rough flat for bending into the desired final part, and through a process of trial and error, the designer would redesign the part until the final part had smooth, finished corners. Alternatively, after folding the part, the colliding or overlapping sections may be trimmed. However, calculating the amount to trim often proves difficult due to the fact that the computation is quite complex.
Simple examples of this problem are illustrated in FIGS. 1-8. Note, the simple problems illustrated in FIGS. 1-8 do not require the complex calculations that a real part requires. These figures are used merely for explanation purposes. FIG. 1 shows a desired user selected 3-D sheet metal part 10. Based upon the specified 3-D part, a simplified flat sheet metal part 11 for bending into the 3-D part 10 can be designed as seen in FIG. 2. Bend lines 12 and 14 are then determined in order to be able to bend the flat into the required shape illustrated in FIG. 1.
However, due to shrinking or stretching of the sheet metal during the bending operation, a collision at a corner occurs and can cause warping of the sheet metal resulting in an undesired finished part. To eliminate the warping, traditionally, if the metal is thin, the colliding section 16 could be ground to make a smooth corner. Another traditional solution to the warping problem is to provide (i.e., cut) a circular relief hole 18 in the flat 11. However, the circular relief hole 18 which may be cut from the flat 11 is generally too large, leaving a gap at the corner formed by the bending. If the gap at the corner is not wanted, an extra step of welding is required to fill the gap left after the part is bent. If the user does not care about the gap, it may be left in the finished part 10. Therefore, it is apparent that a technique is needed for designing the flat sheet metal part 11 such that upon bending it, the corners will be smooth and finished without requiring additional processing operations.
Another traditional solution to this problem requires a rough trial and error process. The designer of the sheet metal part designs the flat 11 through a trial and error process to achieve a folded part 10 having a smooth, finished corner. However, the trial and error process has multiple drawbacks, most notably the use of multiple flats for each trial which must be discarded after one use and the expending of excessive time. Thus, there is a need for a quick and efficient way to design a flat sheet metal part in order to prevent overlap or collisions so as to result in a smooth, finished 3-D sheet metal part.
FIGS. 3 and 4 illustrate a face overlap problem encountered when constructing a box 10 from a sheet metal flat 11. For the box 10 to be finished, faces 21, 23 should come together exactly along a single line when the flat 11 is folded along bend lines 20, 22 without any overlapping section 24. However, because calculating how to design the flat 11 such that faces 21, 23 meet without the overlapping section 24 is complex, typically a designer lets the faces overlap and then determines how much to trim the faces 21, 23 afterwards. Thus, when the flat 11 is folded, and the overlapping section 24 exists, the traditional solution requires unfolding the part and trimming the faces 21, 23 where the overlap occurs. Then the box is refolded to see if the overlap is eliminated. If not, this process is repeated until a finished box results without an overlapping section 24. However, this trial and error process is quite time consuming. Therefore, a need exists for a system which quickly and efficiently designs a flat such that face overlap is eliminated.
FIGS. 5 and 6 illustrate another problem encountered with sheet metal design. When the flat sheet metal part 11 shown in FIG. 6 is bent along bend lines 12 and 14, if the faces 26, 28 (FIG. 5) are non-parallel relative to one another and thus do not touch after bending, gaps are created between faces 26 and 28 thus leaving an undesirable gap in the corner. Traditionally, this problem is solved by having the designer compute the shape of the 2-D flat 11 to compensate for the gap. However, if the designer overcompensates, interference will occur necessitating trim. Or it may even be necessary to fashion another flat 11, if for instance, the interference is so large as to prevent bending. On the other hand, if the designer undercompensates, an undesired gap will still remain. Thus, the calculation necessary for designing the flat having the proper shape to compensate for the non-touching is critical. However, such computations are quite complex. The calculation becomes even more complicated when the thickness of the metal is accounted for, and how much, if any, face contact is desired in the corners. Not only does the calculation take an enormous amount of time and effort, but proper calculation requires the designer to perfectly input the exact geometry of the 3-D part. This can be very difficult and time consuming. Thus, a need exists for a method for designing a flat sheet metal part to create tight, closed corners when sheet metal parts having non-parallel faces are bent without requiring a great effort from the part designer.
Another problem encountered with sheet metal design is illustrated in FIGS. 7 and 8. When the sheet metal part 10 shown in FIG. 7 is bent along bend lines 12, 14 and 32, particularly when the bend along bend line 32 occurs, faces 26 and 30 interfere with each other. The interference or overlap prevents a smooth transition between faces, producing an unfinished or rough, unprofessional looking end product. Traditionally, this problem is solved similarly to the previously described problems through trial and error trimming of both faces or of either face. Alternatively, complicated calculations are necessary to determine the optimum shape of the flat part in order to obtain a smooth corner. Similar to above, the calculation is very complicated, and takes an inordinate amount of time and effort. Thus, there is a need for a system in which a flat part can be designed in order to create a smooth, finished 3-D user specified sheet metal part.